The invention relates to the continuous casting of metals, and more specifically to the continuous casting, directly from liquid metal, of ferritic-type stainless steel strip whose thickness is of the order of a few mm, using the process called xe2x80x9ctwin-roll castingxe2x80x9d.
In recent years considerable progress has been made in the development of processes for casting thin carbon steel or stainless steel strip directly from liquid metal. The process mainly used at the present time is that of casting said liquid metal between two internally cooled rolls, rotating about their horizontal axes in opposite directions and placed opposite one another, the minimum distance between their surfaces being approximately equal to the thickness that it is desired to give the cast strip (for example, a few mm). The casting space containing the liquid steel is defined by the lateral surfaces of the rolls, on which the strip starts to solidify, and by lateral closure plates made of refractory which are applied against the ends of the rolls. The liquid metal starts to solidify on contact with the external surfaces of the rolls, on which solidified xe2x80x9cshellsxe2x80x9d form, arrangements being made for the shells to join together in the xe2x80x9cnipxe2x80x9d, that is to say the region where the distance between the rolls is a minimum.
One of the main problems encountered when manufacturing thin ferritic stainless steel strip by twin-roll casting is that there is a high risk of surface defects called microcracks appearing on the strip. These cracks are small, but they are nevertheless sufficient to make the resulting cold-treated products unsuitable for use. The microcracks form during solidification of the steel and have a depth of about 40 xcexcm and an opening of approximately 20 xcexcm. Their occurrence depends on the conditions, during solidification, under which the steel is in contact with the surface of the rolls over the length of their contact arc. These conditions may be described as having two successive steps. The first step relates to the initial contact between the liquid steel and the surface of the roll, which results in the formation of a solid steel shell at the surface of the rolls. The second step relates to the growth of this shell as far as the nip, where, as mentioned, it joins the shell formed on the other roll in order to constitute the fully solidified strip. The contact between the steel and the surface of the roll is determined by the topography of the surface of the casting rolls, together with the nature of the insert gas and the chemical composition of the steel. All these parameters are involved in establishing heat transfer between the steel and the roll and govern the conditions under which the shells solidify.
Various attempts have been made to develop twin-roll casting processes for obtaining, reliably, strip free of unacceptable surface defects such as microcracks.
The solutions suggested in the case of carbon steel rely on the need for good control of the heat exchange between the steel and the surface of the rolls. In particular, attempts have been made to increase the heat flux extracted from the steel, once it has started to solidify, by the casting rolls. For this purpose, the document EP-A-0 732 163 proposes the use of rolls with a very slight roughness (Ra of less than 5 xcexcm), using these in combination with a steel composition and with production conditions which favor the formation, within the metal, of liquid oxides which wet the steel surface/roll interfaces. With regard to austenitic stainless steel, the document EP-A-0 796 685 teaches the casting of a steel whose Creq/Nieq  ratio is greater than 1.55 so as to minimize the phase changes at high temperature and to carry out this casting by using rolls whose surface includes touching dimples 100-500 xcexcm in diameter and 20-50 xcexcm in depth and by inerting the casting space with a gas soluble in the steel, or a gas mixture composed predominantly of such a soluble gas.
For ferritic stainless steel, the document JP-A-5337612 proposes the casting of a steel having a low carbon content (less than 0.05%) and a low nitrogen content (less than 0.05%) and containing niobium (0.1 to 5%) and titanium. It is also necessary, as the strip leaves the rolls, for it to be cooled at a high rate and then the temperature at which the strip is coiled must be controlled. These production and casting conditions are expensive and demanding, and the particular characteristics of the grades required limit the fields of application of the products thus obtained.
It is an object of the invention to provide a process for casting thin ferritic stainless steel strip whose surface is free of microcracks. Such a process would not require particularly demanding casting conditions to implement it and could be applied to a wide range of grades of such steels.
For this purpose, the subject of the invention is a process for the continuous casting of ferritic stainless steel strip having a thickness of less than or equal to 10 mm directly from liquid metal between two cooled rotating rolls with their axes horizontal, characterized in that:
the liquid metal has the composition in percentages by weight: C %+N %xe2x89xa60.12, Mn %xe2x89xa61, P %xe2x89xa60.04, Si %xe2x89xa61, Mo % less than 2.5, Cr % between 11 and 19, Alxe2x89xa61% and Ti %+Nb %+Zr %xe2x89xa61, the balance being iron and impurities resulting from the smelting;
the xcex3p index of the liquid metal is between 35% and 60%, xcex3p being defined by the formula:
xcex3p=420 C %+470 N %+23 Ni %+9 Cu %+7 Mn %xe2x88x9211.5 Cr %xe2x88x9211.5 Si %xe2x88x9212 Mo %xe2x88x9223 V %xe2x88x9247 Nb %xe2x88x9249 Ti %xe2x88x9252 Al %+189;
the roughness Ra of the surfaces of said rolls is greater than 5 xcexcm;
an inert gas composed of at least 60% by volume of a gas soluble in the steel is used near the meniscus of the liquid metal present between the rolls.
As will have been understood, the invention consists in combining conditions on the composition of the metal, which govern the possibility of forming austenite at high temperature after the metal has solidified, a condition on the minimum roughness of the casting surfaces, and a condition on the composition of the inert gas. By complying with this combination, it is possible to prevent the formation of microcracks on the surface of the strip without correspondingly having to impose too demanding limitations on the casting process and without excessively restricting the fields of application of the products which will be manufactured from the cast strip.